The invention relates to a nuclear reactor with improved efficiency capable of better utilization of the fuel material of the core arrays.
Nuclear reactors cooled and moderated by pressurized light water comprise a vessel containing the reactor core immersed in pressurized water filling the vessel. The core of the reactor comprises tall arrays relative to their cross-section arranged vertically and side by side. The arrays themselves consist of bundles of fissile fuel rods in contact by their external surface with the cooling water of the reactor.
For the operation of the reactor, an assembly of control rods associated with certain arrays of the core is used. These control rods consist of parallel bars of strongly neutron-absorbing material which can be moved vertically within guide tubes replacing some fuel rods in the arrays forming the core.
One of the major problems involved in the operation of nuclear reactors is to obtain high efficiency as regards the use of the nuclear fuel of the arrays. This fuel generally consists of uranium in the form of uranium oxide containing fertile uranium 238 preponderantly and a quantity of fissile uranium 235 which varies as a function of the enrichment of the fuel.
During the operation of the reactor, the fissile fuel is consumed so that it is necessary to replace at least a part of the core arrays of the reactor after a certain period of operation.
The cost of the operations to enrich, recharge, replace the used fuel and withdraw it is very high, so that it is desirable to make the best possible use of the fuel introduced into the reactor core in order to improve the economic operating conditions of the reactor.
It is particularly attempted to effect the most complete possible combustion of the uranium contained in the material of the arrays. By improving the combustion of the uranium, it is possible either to prolong the useful life of the core for a given initial charge of fissile uranium, or to reduce the initial charge of fissile uranium in the core for a given useful life. In the former case, the operating costs of the nuclear reactor are reduced by effecting recharges at longer intervals of time. In the latter case, it will be possible, for example, either to reduce the volume and the total mass of the fuel rods of the core, or again to use a fuel with a lower degree of enrichment. In this way the cost of the fuel charge will be reduced.
In order to operate the reactor, that is to say in order to regulate the reactivity of the core, neutron-absorbing materials are used either in the form of control rods which are inserted into the core of the reactor, or in the form of elements dissolved in the cooling and moderating water of the reactor. After the core is charged, its reactivity is high, so that it is necessary to use absorbing materials in increased quantity for the operation of the reactor. For example, clusters of rods containing consumable poisons are introduced into the guide tubes of some arrays of the core, or again neutron-absorbing poisons are introduced in considerable quantity into the cooling water.
When the excess reactivity decreases due to the exhaustion of the fuel, the concentration of the neutron-absorbing poisons which are dissolved is decreased correspondingly. These neutron-absorbing poisons, which are necessary for the operation of the reactor in its initial state, are expensive in themselves and reduce the energetic efficiency of the fissile fuel contained in the core.
It has been proposed to utilize the excess reactivity of the core in its initial state to produce a fissile fuel (plutonium 239) from the uranium 238 contained in the fuel of the arrays. To do this, the neutron energy spectrum in the core is shifted towards the high energies, by reducing the ratio of the volume of moderator to the volume of fuel in the core, during the first part of the fuel cycle. When the excess reactivity of the fuel becomes virtually zero, the ratio of volume of moderator to volume of fuel is restored to a value permitting the neutron spectrum to be restored to its customary zone for pressurized water nuclear reactors. The neutrons are then said to be "thermal" or "slow". This has the effect of producing a fresh excess of reactivity, which permits the period of use of the fuel to be prolonged.
The ratio of moderator volume to fuel volume is influenced by introducing into the first part of the fuel cycle, bars of neutron-transparent material within some guide tubes of the core arrays. In this way the water contained by these guide tubes is expelled and the volume of moderator in the core is reduced by this amount.
To obtain an appreciable effect, it is necessary to displace approximately 20% of the cooling water during almost 60% of the useful life of the core. To do this, it is necessary to use a very large number of neutron-transparent rods introduced into all the guide tubes of the core arrays, with the exception of those used for the guidance of the absorbing control rods of the reactor.
This considerably complicates the conception and the design of the reactor.
In fact, all the equipments containing the core of the reactor must be dimensioned so as to be able to perform the guidance above the core and the control in translation of the spectrum variation rods. This conception therefore dictates the insertion of a large number of guide tubes in that part of the internal equipments through which the heat-laden water normally escapes, to the detriment of the water balance of the reactor. It is therefore necessary to adopt a fresh conception of the circulating of the coolant in the reactor vessel. Moreover, the control in translation of these clusters necessitates the location on the cover of the vessel of a very large number of control mechanisms which must be interposed with the existing mechanisms of the control clusters permitting the running of the reactor. All these demands lead particularly, for equal power, to an increase in the dimensions of the reactor vessel compared to a conventional reactor.
Moreover, a shift of the neutron spectrum towards the high energies involves an increased loss of neutrons outside the reactor and greater "embrittlement" of the steel of the reactor vessel.